High voltage transformer



Oct. 25, 1960 G. CAMILLI 2,958,058

HIGH VOLTAGE TRANSFORMER Filed March 21, 1957 3 Sheets-Sheet. 1

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Oct. 25, 1960 G. CAMILLI HIGH VOLTAGE TRANSFORMER 3 Sheets-Sheet 2 Filed March 21, 1957 [72 en G'u JZYYJ CCL 2.5, 1960 G. CAMILLI 2,958,058

HIGH VOLTAGE TRANSFORMER Filed March 21, 1957 5 Sheets-Sheet 3 LINE United States Patent HIGH 'VOLTAGE TRANSFORMElR Guglielmo Camilli, Pittsfield, Mass., assignor to General vElectric Company, a corporation of New York Filed Mar. 21, 1957, Ser. No. 647,719

1 Claim. (Cl. 336-174) ',lhis invention relates to Yhigh voltage electrical induction apparatus, vand more in particular to an improved high voltage Current transformer and a method for ,making the same.

` High voltage current transformers of one type, com- .rnonly referred to as the eye-bolt type, are `comprised of an annular high voltage winding of one or more conductor turns interlinking one or more toroidal low voltage .secondary windings wound on annular magnetic cores.

The windings are generally enclosed in a dielectric uid yfilled tank, and a pair of leads extend from the high .voltage winding upwardly from the top of the tank,

through an insulating ceramic shell, and are connected to a suitable terminal assembly disposed at the upper end of ,the ceramicshell. The leads from the low voltage .winding or windings may be connected to low voltage bushings disposed in the side Walls of the tank.

In the fabrication of transformers of this type, the

turns of the high voltage winding are interlinked with the low voltage windings, and in order to provide the .necessary insulation for the high voltage winding, the high voltage winding and its associated leads are then manually wrapped with an insulating tape, such as paper tape. Following the taping of the high voltage winding,

a ground shield may be provided on the high voltage .winding The manual taping of a high voltage winding lin Asome cases must be accomplished after the high voltage winding has been interlinked with the other windings. Thus, as a result of the coniguration of the high voltage .winding .and the low voltage winding, it is diiicult and .time consuming to properly insulate the windings, and

.the cost of ythe transformer is thereby substantially increased.

It is therefore an object of this invention to provide improved high voltage transformer and a method for making the same.

It is also an object to provide a high voltage current transformer having such a structure that the necessity for manual insulation .of the windings is minimized or eliminated.

improved method for fabricating a high voltage current transformer, the method being characterized by the Obyiation o-f the necessity for manual insulation of the windings.

Briey stated, in accordance with one aspect of my invention, I provide a high voltage current transformer having a high voltage winding comprising a loop of a flexible cable. The cable has one or more central conductors surrounded by a exible insulating material. As

2,958,958 Patented' Oct. 25, 1960 a WCC will be described in more detail in the following disclosure, the insulating material is preferably comprised of a plurality of layers of insulating tape, such as paper tape, having a substantial degree of llexibility axially with respect to the cable. Electrostatic equalizing means are imbedded in the insulating material surrounding the central conductor in order to provide a desirable lvoltage gradient across the insulating material, and to obtain fairly uniform voltage distribution on a hollow ceramic insulator when the cable is assembled and enclosed within the insulator. The high voltage winding is interlinked with one or more low voltage toroidal windings according to the usual practice.

While the specication concludes with a claim particularly pointing out and distinctly claiming the subject matter which l regard as my invention, it is believed that the invention will be better understood from the following description taken in connection With the accompanying drawing in which:

Fig. l is a partially cross-sectional View of a high voltage current transformer and illustrating a high voltage or primary winding according to my invention,

Fig. 2 is an enlarged partially cross-sectional view of the high voltage winding of Fig. l,

Fig. 3 is a cross-sectional View of a high voltage winding according to my invention,

Fig. 4 is a view of a modified form of the primary winding of Fig. 2 and illustrating means for providing a high voltage winding having a plurality of turns,

Fig. 5 is a cross-sectional view lof a modified high voltage current transformer according to my invention, and

Fig. 6 is an equivalent capacitance diagram of the transformer of Fig. 5.

Referring now to the drawing, and more in particular to Fig. 1, therein is illustrated a current transformer of the eye-bolt type fabricated according to my invention. A exible cable 10 is provided having a central loop portion 11 joining relatively straight portions 12. The loop portion 11 comprises the primary or high voltage winding of the transformer. The high voltage winding is disposed in a dielectric fluid filled tank 13, and therein is interlinked with one or more low voltage toroidal secondary windings 14. The low voltage windings 14 may be of the conventional type wound on annular magnetic cores, and it will be obvious that the number of such coils employed may be varied without departing from the spirit or scope of this invention.

A ceramic insulating shell 15 is disposed on the top of the tank 13, and the straight lead portions 12 of the flexible cable extend upwardly through the top of the tank 13 and through the ceramic shell 15 and are connected in a conventional manner to suitable terminals 16 disposed in upper terminal assembly 17 at the upper end of the ceramic shell 15. The low voltage winding leads may be connected to low voltage bushings 18 disposed in the side walls of the tank 13.

Referring now to Fig. 2 the flexible cable 10 is therein iliustrated as being comprised of a central conductor 20 surrounded by a exible insulating material 2.1. While other flexible cables may be employed, the cable l0 is preferably of the type disclosed in U.S. Letters Patent 2,607,824r which issued on August 19, 1952 on an application of G. Camilli et al. and assigned to the assignee of the present invention. The cable of the Camilli et al. patent is comprised of a plurality of layers of crepe paper tape spirally wound about a conductor, the crepe paper tape having two crepings substantially perpendicular to each other. In this type of cable, the insulating material has a substantial degree of stretchability in the axial direction with respect to the cable, thereby permitting the cable to be bent on a small enough radius to enable its use as a high voltage winding of a high voltage current transformer. Referring still to Fig. 2, the insulating material 21 is preferably comprised of a plurality of -la'yers 'Z2`of insulating material separated by electrostatic equalizing'layers 23. Abe comprised of flexible metallic shielding, or any other yflexible conductive material. The cable 12 is also prefer- 'ably provided with an outer conductive shielding 24 TheV equalizing layers may surrounding the insulating material for the purpose of providing a ground shield for the primary winding.

From the previous disclosure, it is obvious that the cable 12 may be formed in long lengths by conventional cable forming techniques, the equalizing layers 23 being imbedded in the insulating material during the forming of the cable, the desired lengths of cable for use in a current transformer may be severed from the long length that 'is ythus formed. In this manner, the necessary insulation and shielding for the high voltage winding is provided prior to the forming of the winding, and manual insulation thereof is minimized or entirely eliminated.

Referring still to Fig. 2, the insulation and shielding of the straight lead portions 12 of the llexible cable is vremoved in steps, with progressively decreasing insula- -tion thickness toward the ends 25 of the cable.

This results in the ends of the equalizer layers being progressively increasing distances from the ends of the cable radially outwardly of the conductor 20, or in other words,

`the equalizers are progressively longer toward the central strip of insulating material 2"! may be provided to prevent contact between the ends of the outer shielding 24.

The layers of electrostatic shielding in the cable insulation act as a capacitance voltage divider, and distribute the electric stress throughout the cable in inverse proportion to the capacities between shielding layers. By stepping the electrostatic shielding the area of the outer capacitances is reduced, and the electric stress is thereby more nearly equalized throughout the cable. The equalization of stress in the insulation makes possible the use of a flexible cable in a high voltage current transformer, since the equalizing of stress permits a reduction in the radial thickness of the insulation, and as a result a cable having sufficient flexibility to form a loop within the confines of a reasonably sized transformer structure may be readily provided.

Referring now to Fig. 3, therein is illustrated a crosssectional view of a cable loop for a typical high voltage winding of a current transformer according to my invention. In this illustration, the cable has four concentric layers of insulation 30, 31, 32 and '33 surrounding a conductor 34. (The lower numbered layers of insulation being closer to the conductor.) Between the Vlayers 30, 31, 32 and 33 are thin layers of flexible conducting material 35', 36 and 37 respectively. A conducting shield 38 surrounds the outer insulation layer 33. As in the high voltage winding of Figs. l and 2, the ends of the equalizer layers and insulation layers are stepped, with the layers toward the conductor being progressively longer. The insulation layers have the same length as the next adjacent radially inward equalizer layer, and the outer shield is shorter than the insulation layer 33. The distance between the end of the outer shield and the end of inner insulation layer 3i) is referred to as the scarf. In tests on the winding loop of Pig. 3, (see Table I) with the exposed portions of the insulation layers 30, 31, 32 and 33 being equal, it was shown that the voltage stress in the layers of insulation is not appreciably affected by either the length of the cable or by the length of the scarf. In Table I, the voltages V1, V2, V3, and V4 are respectively the voltages across the insulation layers 30, 31, 32 and 33, and the total length of the cable is the distance around the loop between the ends of inner insulation layer 30. (See Table I.)

In view of the slight independence of voltage stress on cable and scarf length, the stepping of the insulation and conducting layers in the scarf may be designed primarily from the standpoint of obtaining uniform voltage distrib-ution on the ceramic insulator which surrounds the ends of the loop.

Table l Scarf Voltage in Percent of Total Voltage Length Between the Conductor and Outer Total Length of on each Shield Cable (Feet) end (Inches) Y V1 Vn Vs V4 4 y 12 33.9 24. 5 21. 2 20. 3 15 3l. 5 23.7 2l. 8 22. 9 6 12 36. 6 25. 2 20. 4 17. 8 15 35. 4 24. 8 20. 8 18. 9 8 12 37. 8 25. 4 20. 0 16. 9 15 36. 9 25. 2 20. 3 17. 7 12 12 38. 9 25.5 19. 6 16. 0 15 38.4 25.4 19.7 16.4

Referring now to Fig. 4, therein is illustrated a modication of the winding of Fig. 2 in which the cable 10 is provided with two central conductors 40 and 41, the conductors 40 and 41 being provided with separate insulation 42 one end of each of the conductors 40 and 41 are electrically joined by any suitable means at the ends of the cable 12, so that they are serially connected. -By this means, the primary winding may be provided with two turns. It will be obvious, of course, that any number of turns may be provided in the primary Winding in a similar manner by providing a plurality of insulated conductors in the cable, and serially connecting the ends of the conductors.

In order to fabricate the current transformer of my invention, the desired number of low voltage toroidal secondary coils are wound on annular magnetic cores, and these low voltage windings are taped with suitable insulating tape, and may also be provided with ground shields. Then a loop of the previously disclosed flexible cable is interlinked with the low voltage windings. If more than one primary turn is desired, the flexible cable may be provided with a plurality of central conductors, and these central conductors then serially Vconnected in the manner illustrated in Fig. 4.

Referring now to Fig. 5, in a modified form of the current transformer of my invention, a primary winding 10 consisting of a loopv of cable is provided, the winding being of the same construction as the primary winding of Figs. l and 2. The loop 11 of the high voltage winding is interlinked with a toroidal low voltage winding 50. According to conventional practice, the low voltage may be wound on an annular magnetic core S1. Theleads 52 of the low voltage winding extend downwardly through a ceramic insulator 53, and are connected to low Voltage terminals on the transformer (not illustrated).V The 10W voltage winding is adapted, in operation, to b e substan- `tially at ground potential. Insulation 54 is provided surrounding the low voltage winding, the insulation also extending into the ceramic insulator 53. The insulation 54 may be formed of manually wrapped layers of nsulating tape. A metallic shield 55 is provided surrounding the insulation 54, `and the shield 55 may be provided with a gap 56 to prevent theV llow of circulating currents therein.

The interlinked high and low voltage windings are surrounded by a tank 57, the tank extending between the ceramic insulators 15 and 53 and being sealed to the insulators so that a dielectric fluid may be contained therein.

Electrical connection is made between the outer shield 24 of the high voltage winding and the outer shield 55 of the low voltage winding, for example, by a lead i60. Elec trical connection is also made between the tank 57 and outer shield 24 of the high voltage winding, such as by a lead 61.

The equivalent capacitive circuit of the transformer of Fig. 5 is illustrated in Fig. 6, wherein capacitances C1 and C2 are illustrated serially connected between a power line and ground reference. Capacitor C1, connected to the power line, represents the capacitance between the central conductor 20 of the high voltage winding and the outer shield Z4 of the high voltage winding. 'Ihe insulation 21 of the high voltage winding corresponds to the dielectric of capacitor C1. Capacitor C2, connected to the ground reference, represents the capacitance between the outer shield 55 of the low voltage winding and the low voltage winding 50. The dielectric of capacitor C2 represents the insulation 54 of the low voltage winding.

From the illustration of Fig. 6, it is seen that the proportion of the total line voltage that is developed across the high voltage insulation 21 and the low voltage insulation 54 depends upon the relative values of capacitors `C1 and C2, C1 being the conductor to outer shield capacitance of the primary winding and C2 being the conductor to outer shield capacitance of the low voltage winding,

The transformer arrangement of Fig. 6 while having the advantages of the transformer of Fig. 1, has the additional advantage that it lends itself to higher voltage application than the transformer of lFig. 1. Thus, in the arrangement of Fig. 6, the required high voltage winding insulation depends upon the proportion between C1 and C2, and thus it is not necessary to insulate the cable for the full line voltage. For example, the cable insulation may be rnade suitable to withstand only one-half of the full line voltage, and the low voltage winding would also be insulated to withstand only one-half of the line voltage, if the capacitances C1 and C2 are equal. The windings of the transformer of Fig. 5 may be made by the same method as the windings of the transformer of Fig. 1.

It will be understood, of course, that while the forms of the invention herein shown and described constitute preferred embodiments of my invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications thereof. It will also be understood that the words employed are words of description rather than words of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed, and it is aimed in the appended claim to cover all such changes as fall within the -true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

A high voltage current transformer comprising a tank, a single bushing supported by said tank, a toroidal low voltage winding in said tank, a high voltage winding formed from a length of cable having pre-wound insulation that has sucient flexibility axially of said cable to allow said cable to be bent into a loop that passes through said toroidal low voltage winding within the confines of said tank, end portions of said cable forming relatively straight leads from said high voltage winding that extend into said single bushing only, said cable having at least one central conductor, the pre-wound insulation of said cable comprising a plurality of layers of insulating t-ape having two crepings perpendicular to each other, a plurality of flexible electrostatic equalizers imbedded in said insulation and concentric with said conductor, said equalizers being insulated from said conductor and from each other, and the ends of said equal- `izers being at progressively increasing distances from the ends of said cable radially outward of said conductor.

References Cited in the file of this patent UNITED STATES PATENTS 1,561,204 Beers July 26, 1923 1,701,279 Silbermann Feb. 5, 1929 2,331,106 Camilli Oct. 5, 1943 FOREIGN PATENTS 204,706 Great Britain Oct. 9, 1924 416,695 Great Britain Sept. 19, 1934 739,497 Great Britain NOV. 2, 1955 741,162 Great Britain Nov. 30, 1955 

